High-statistics $π^{-}π^{-}$ HBT data for non-central Au + Au collisions at 1.23$A$ GeV, measured with HADES at SIS18/GSI, are presented. The three-dimensional emission source is studied in dependence on pair transverse momentum and centrality. A tilt of the source relative to the beam axis is observed. The spatial extension and the tilt magnitude of the source decrease with transverse momentum. The spatial extension decreases and the tilt magnitude increases going from central to peripheral collisions. The derived eccentricity perpendicular to the beam axis fits well to the initial nucleonic overlap region at high transverse momentum.
Understanding the origins of collective, fluid-like behavior in ultrarelativistic nuclear collisions constitutes one of the biggest open challenges in the field. In this Letter, it is argued that certain features in the multiplicity dependence of the source sizes extracted using pion interferometry may be understood quite naturally if small systems evolve hydrodynamically at sufficiently large multiplicities. Interferometry may therefore provide a baseline for probing and constraining the nature of collective behavior in nuclear collisions.
Interferometry correlations in central p+Pb collisions
在3+1维粘性流体动力学模型中，利用Glauber Monte Carlo模型的初始条件，给出了5.02TeV中心p+Pb碰撞中π介子的干涉关联.计算了关联函数作为π对快度的函数。所提取的干涉半径显示出弱的快度依赖性，反映了π介子分布缺乏增强不变性。发现$out$和$long$方向之间的交叉项是非零的。在流体动力学模型中得到的结果是在公平的协议与最近的数据的ATLAS合作。
